Lambda syntax and behavior in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a lambda function in Python?

A lambda function is a small anonymous function defined with the lambda keyword. It can take any number of arguments but has only one expression, which is returned automatically.

beginner

How do you write a lambda function that adds 5 to its input?

You write it as lambda x: x + 5. This means the function takes one argument x and returns x + 5.

beginner

Can a lambda function contain multiple expressions or statements?

No, a lambda function can only contain a single expression. It cannot have multiple statements or commands like loops or assignments.

intermediate

How does a lambda function differ from a regular function defined with def?

A lambda function is anonymous (has no name) and is limited to one expression. A regular function can have multiple statements, a name, and more complex behavior.

beginner

What is the output of this code?

add = lambda x, y: x + y
print(add(3, 4))

The output is 7. The lambda function adds the two inputs 3 and 4 and returns the sum.

What keyword is used to create a lambda function in Python?

A lambda

B def

C func

D function

The lambda keyword is used to create anonymous functions in Python.

Which of the following is a valid lambda function syntax?

A def lambda(x, y): return x + y

B lambda (x, y) { return x + y }

C lambda x, y: x + y

D lambda x, y { x + y }

Option C is the correct syntax for a lambda function in Python.

What will this lambda function return?

lambda x: x * 2

A Double the input value

B Half the input value

C Always returns 2

D Returns the input unchanged

The lambda function multiplies the input by 2, so it returns double the input.

Can a lambda function have multiple statements inside it?

A Only if using semicolons

B Yes, multiple statements separated by commas

C Yes, if enclosed in parentheses

D No, only one expression is allowed

Lambda functions are limited to a single expression and cannot contain multiple statements.

What is the main use of lambda functions?

A To define classes

B To create small anonymous functions quickly

C To replace all regular functions

D To write multi-line functions

Lambda functions are mainly used for quick, small anonymous functions, often as arguments to other functions.

Explain what a lambda function is and how it differs from a regular function defined with def.

Think about the size and naming of the function.

Write a lambda function that takes two numbers and returns their product. Then explain how it works.

Use the format: lambda arguments: expression

Practice

1. What does a lambda function in Python do?

easy

A. Creates a small anonymous function with a single expression

B. Defines a class with methods

C. Declares a variable

D. Imports a module

Solution

1. Step 1: Understand lambda purpose

   A lambda function is a quick way to create a function without a name that returns the result of one expression.

2. Step 2: Compare options

   Only Creates a small anonymous function with a single expression describes this behavior correctly. Other options describe unrelated Python features.

3. Final Answer:

   Creates a small anonymous function with a single expression -> Option A

4. Quick Check:

   Lambda = anonymous single-expression function [OK]

Hint: Lambdas are short unnamed functions with one expression [OK]

Common Mistakes:

• Thinking lambda can have multiple statements
• Confusing lambda with class or variable definitions
• Assuming lambda needs a name

2. Which of the following is the correct syntax for a lambda function that adds two numbers x and y?

easy

A. lambda (x, y) { return x + y }

B. lambda x, y: x + y

C. def lambda(x, y): return x + y

D. lambda x, y => x + y

Solution

1. Step 1: Recall lambda syntax

   In Python, lambda syntax is: lambda parameters: expression

2. Step 2: Check each option

   lambda x, y: x + y matches correct syntax. Options B and D use other language styles. def lambda(x, y): return x + y wrongly uses def with lambda.

3. Final Answer:

   lambda x, y: x + y -> Option B

4. Quick Check:

   Correct lambda syntax = lambda params: expression [OK]

Hint: Remember: lambda params colon expression [OK]

Common Mistakes:

• Using parentheses around parameters in lambda
• Trying to use curly braces or return keyword
• Mixing lambda with def syntax

3. What is the output of this code?

func = lambda x: x * 2
print(func(5))

medium

A. 10

B. 25

C. SyntaxError

D. None

Solution

1. Step 1: Understand the lambda function

   The lambda takes input x and returns x multiplied by 2.

2. Step 2: Calculate func(5)

   func(5) = 5 * 2 = 10

3. Final Answer:

   10 -> Option A

4. Quick Check:

   5 * 2 = 10 [OK]

Hint: Multiply input by 2 as lambda defines [OK]

Common Mistakes:

• Confusing multiplication with addition
• Expecting syntax error due to lambda
• Thinking lambda returns None by default

4. Find the error in this lambda function:

double = lambda x: return x * 2
print(double(4))

medium

A. No error, code runs fine

B. Missing parentheses around x

C. Lambda cannot multiply numbers

D. Using 'return' inside lambda causes SyntaxError

Solution

1. Step 1: Check lambda syntax rules

   Lambdas cannot contain statements like 'return'; they only have an expression.

2. Step 2: Identify error in code

   The code uses 'return' inside lambda, which is invalid and causes SyntaxError.

3. Final Answer:

   Using 'return' inside lambda causes SyntaxError -> Option D

4. Quick Check:

   Lambda disallows 'return' keyword [OK]

Hint: Lambdas never use 'return' keyword [OK]

Common Mistakes:

• Adding 'return' inside lambda
• Thinking parentheses are mandatory around parameters
• Believing lambda can't do math operations

5. You want to create a list of squares for numbers 1 to 5 using a lambda inside map. Which code correctly does this?

hard

A. list(map(lambda x: x*2, range(1, 5)))

B. map(lambda x: x**2, [1, 2, 3, 4, 5])

C. list(map(lambda x: x**2, range(1, 6)))

D. list(map(lambda x: x**2, range(5)))

Solution

1. Step 1: Understand the goal

   Create squares of numbers 1 to 5 inclusive, so numbers are 1,2,3,4,5.

2. Step 2: Check each option

   list(map(lambda x: x**2, range(1, 6))) uses range(1,6) which includes 1 to 5 and squares each number correctly.
   list(map(lambda x: x*2, range(1, 5))) doubles numbers and uses range(1,5) which excludes 5.
   map(lambda x: x**2, [1, 2, 3, 4, 5]) returns a map object, not a list.
   list(map(lambda x: x**2, range(5))) uses range(5) which is 0 to 4, not 1 to 5.

3. Final Answer:

   list(map(lambda x: x**2, range(1, 6))) -> Option C

4. Quick Check:

   Squares 1-5 with map and lambda [OK]

Hint: Use range(1,6) and lambda x: x**2 inside list(map()) [OK]

Common Mistakes:

• Using range(5) which starts at 0
• Forgetting to convert map to list
• Using wrong lambda expression like x*2